1. Field of the Invention
The present invention relates to an apparatus and method to compensate a color carrier of an image signal, and more particularly, to an apparatus and method to automatically compensate for a phase deviation of a color carrier of an image signal when converting an analog image signal to a digital image signal.
2. Description of the Related Art
Referring to FIG. 1, in order to convert an input analog image signal to a digital image signal, a conventional image processing system includes a phase-locked loop (PLL) unit 100, an analog-to-digital converter (ADC) 110, a comb filter 120, a color signal processor 130, and a format converter 140. The image processing system shown in FIG. 1 is referred to as a video decoder.
The PLL unit 100 includes a PLL1 101 and a PLL2 102 and generates signals by phase locking an input system clock signal to desirable frequencies. In other words, the PLL1 101 generates a sampling clock signal suitable for the ADC 110, and the PLL2 102 generates a fixed subcarrier signal to be used by the color signal processor 130.
The ADC 110 converts the input analog image signal into the digital image signal in response to the sampling clock signal provided from the PLL1 101 and outputs the digital image signal to the comb filter 120. The comb filter 120 divides the digital image signal into a luminance signal Y and a color signal C. The color signal processor 130 compensates for a phase deviation of a color carrier of the color signal C received from the comb filter 120 based on the subcarrier signal received from the PLL2 102. The format converter 140 converts the color signal received from the color signal processor 130 and the luminance signal Y received from the comb filter 120 into an appropriate output format and outputs a result indicative thereof. An image signal output from the format converter 140 is the digital image signal.
As described above, when the conventional image processing system converts the input analog image signal to a digital image signal, the color signal processor 130 compensates for the phase deviation of the color carrier, i.e., a reference signal for color, using the fixed subcarrier frequency generated from the PLL2 102. Accordingly, when the phase deviation of the color carrier included in an input image signal goes beyond a predetermined standard deviation range due to changes in an external environment, or when the phase of a subcarrier goes beyond a standard tuning range due to a deviation of a load cap of an oscillation device, the color signal processor 130 cannot exactly compensate for the phase deviation of the color carrier; thereby generating problems, such as the color of the output image signal may be changed, or a black and white image signal may be displayed. Changes in the external environment may include defects in a recording medium, such as a tape.